1. Field of the Invention
The present invention relates to a stepping motor of a cylindrical configuration applicable to a small apparatus, such as a photographing apparatus or a mobile phone.
2. Related Background Art
A stepping motor having a reduced diameter about an output shaft and an enhanced output level has been proposed (refer to Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356)). FIG. 10 shows an exploded perspective view of the stepping motor. FIG. 11 shows a sectional view of an assembled stepping motor taken along its axis.
FIGS. 10 and 11 show a rotor 201 having a cylindrical shape which is formed of permanent magnet divided into four parts in a peripheral direction so as to alternately magnetize the parts into different polarities, a first coil 202 arranged adjacently to the rotor 201 in an axial direction, a second coil 203 similarly arranged adjacently to the rotor 201 in the axial direction, a first stator 204 that is formed of a soft magnetic material and excited by the first coil 202, and a second stator 205 that is formed of a soft magnetic material and excited by the second coil 203. The first stator 204 includes first outer magnetic pole portions 204A and 204B that are opposed to an outer peripheral surface of the rotor 201 so as to have a clearance therebetween and first inner magnetic pole portions 204C and 204D that are opposed to an inner peripheral surface of the rotor 201 so as to have a clearance therebetween. The second stator 205 includes second outer magnetic pole portions 205A and 205B that are opposed to the outer peripheral surface of the rotor 201 so as to have a clearance therebetween and second inner magnetic pole portions 205C and 205D that are opposed to the inner peripheral surface of the rotor 201 so as to have a clearance therebetween.
The rotor 201 is fixed to an output shaft 206. The output shaft 206 is rotatably supported by a bearing portion 204E of the first stator 204 and a bearing portion 205E of the second stator 205. The first stator 204 and the second stator 205 are held by a coupling ring 207 formed of a non-magnetic material so as to have a predetermined clearance between the two stators.
With the stepping motor of the above arrangement, a current flowing through the first coil 202 is reversed in direction to switch the polarities of the first outer magnetic pole portions 204A and 204B and the polarities of the first inner magnetic pole portions 204C and 204D, and a current flowing through the second coil 203 is similarly reversed in direction to switch the polarities of the second outer magnetic pole portions 205A and 205B and the polarities of the second inner magnetic pole portions 205C and 205D. The rotor 201 is thus rotated.
In the stepping motor of this type, magnetic flux lines generated by energization of the coil pass from the outer magnetic pole portion to the inner magnetic pole portion opposed thereto or from the inner magnetic pole portion to the outer magnetic pole portion opposed thereto, thereby acting efficiently on each magnet composing the rotor arranged between the outer magnetic pole portion and the inner magnetic pole portion. Further, a distance between the outer magnetic pole portion and the inner magnetic pole portion that are opposed to each other can be set as being approximately as large as the thickness of the magnet having a cylindrical shape, thereby making it possible to reduce the resistance of a magnetic circuit formed by the outer magnetic pole portion and the inner magnetic pole portion. As the resistance of the magnetic circuit becomes smaller, more magnetic flux lines can be generated with a smaller amount of current, resulting in the enhanced output level.
Alternatively, an improved structure of the above stepping motor has been disclosed (refer to Japanese Patent Application Laid-Open No. H10-229670). In the structure, an inner magnetic pole portion is formed into a cylindrical shape, an output shaft (rotor shaft) inserted into an inner portion of the inner magnetic pole portion is formed of a soft magnetic material and attached to a stator (composed of the inner magnetic pole portion and the outer magnetic pole portion), and a bearing that rotatably supports the output shaft is formed of a soft magnetic material. According to the proposed structure, the output shaft is also included in a magnetic circuit, enhancing the output of the motor.
However, the motor disclosed in Japanese Patent Application Laid-Open No. H10-229670 has a problem in that magnetic flux lines generated by energization of a first coil adversely affects a second coil, a second outer magnetic pole portion, and a second inner magnetic pole portion via the output shaft formed of a soft magnetic material, and magnetic flux lines generated by energization of a second coil adversely affects a first coil, a first outer magnetic pole portion, and a first inner magnetic pole portion via the output shaft formed of a soft magnetic material, resulting in unstable rotation.
The motors disclosed in Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356) and Japanese Patent Application Laid-Open No. H10-229670 both require a predetermined clearance between the inner periphery of the magnet and the inner magnetic pole portion opposed thereto, and management of the clearance during manufacture causes an increase in manufacturing cost. Further, the stator needs to include the inner magnetic pole portion and the outer magnetic pole portion that are formed into the cylindrical shape, and it is difficult to integrally structure those portions from the viewpoint of parts manufacturing. Further, in the case where those portions are separately manufactured and then integrally assembled, the number of necessary parts becomes large, causing an increase in manufacturing cost.
Further, recently, there is a particular demand for highly accurate indexing. To achieve this, it is necessary to enhance the resolution of the motor, and to make the magnetization separation pitch of the magnet finer, increasing the number of magnetization poles. However, due to the manufacturing factors of the magnetized yoke and from the viewpoint of formation of a strong magnetic field for magnetization, it is rather difficult to increase the number of magnetization poles of a magnet; usually, magnetization is effected at a pitch of approximately 1 mm. Thus, there is a demand for a magnet which is easy to manufacture, which has a strong magnetic force, and which exhibits a minute magnetization pitch.